Im limit state design, under-reinforced section is one in which

tensile strength in steel reaches yield value while maximum compressive strain in concrete is less than its ultimate crushing strain

maximum tensile stress in steel reaches its permissible value while maximum compressive stress in concrete is less than its permissible value

maximum compressive strain in concrete reaches the ultimate crushing value while tensile strain in steel is less than its yield value

maximum compressive stress in concrete reaches its permissible value while tensile stress in steel is less than its permissible value